U.S. Department of the Interior

U.S. Geological Survey

Data Series 895

Geochemical and Petrographic Data for Intrusions Peripheral 

to the Big Timber Stock, Crazy Mountains, Montana
COVER. Prominent Eocene sill along the ridge crest, above rocks of the Fort Union Formation, 

4 kilometers northeast of Gobblers Knob, Montana. (Photo by Anna B. Wilson, U.S. Geological 

Survey, 1992.)
Geochemical and Petrographic Data for 

Intrusions Peripheral to the Big Timber 

Stock, Crazy Mountains, Montana

By Edward A. du Bray, Anna B. Wilson, and Bradley S. Van Gosen

Data Series 895

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

du Bray, E.A., Wilson, A.B., and Van Gosen, B.S., 2015, Geochemical and petrographic data for intrusions 

peripheral to the Big Timber stock, Crazy Mountains, Montana: U.S. Geological Survey Data Series 895, 19 p.,

http://dx.doi.org/10.3133/ds895.

ISSN 2327-638X (online)
iii

Contents

Introduction.....................................................................................................................................................1

Analytical Methods........................................................................................................................................1

Data Fields.......................................................................................................................................................1

Geochemical Characteristics ......................................................................................................................3

Major Oxide Data..................................................................................................................................3

Trace-Element Data..............................................................................................................................8

Petrographic Characteristics ....................................................................................................................10

Synthesis.......................................................................................................................................................11

Acknowledgments.......................................................................................................................................11

References Cited..........................................................................................................................................11

Appendix 1. Status and Treatment of Samples of Sills and Plugs Peripheral to the 

Big Timber Stock, Crazy Mountains, Montana..........................................................................14

Appendix 2. Geochemical and Petrographic Data for Samples of Sills and Plugs 

Peripheral to the Big Timber Stock, Crazy Mountains, Montana...........................................15

Figures

1. Regional geologic setting for the Crazy Mountains, Montana, showing the 

Big Timber stock, associated radial dikes, and peripheral sills and plugs .........................2

2. Variation diagram showing relative alumina and alkali saturation of igneous 

rocks peripheral to the Big Timber stock, Crazy Mountains, Montana, as a 

function of molar major-oxide compositions............................................................................6

3. Variation diagram showing FeO/(FeO+MgO) values for igneous rocks peripheral 

to the Big Timber stock, Crazy Mountains, Montana, relative to boundaries 

between ferroan and magnesian rocks....................................................................................7

4. Total alkali-silica variation diagram showing compositions of igneous rocks 

peripheral to the Big Timber stock, Crazy Mountains, Montana..........................................7

5. Variation diagram showing Na2O+K2O-CaO values versus SiO2 content among 

samples of igneous rocks peripheral to the Big Timber stock, Crazy Mountains, 

Montana .........................................................................................................................................8

6. Variation diagrams showing abundances of major oxides in igneous rocks 

peripheral to the Big Timber stock, Crazy Mountains, Montana..........................................9

7. Trace-element, tectonic setting–discrimination variation diagram showing 

the composition of igneous rocks peripheral to the Big Timber stock, Crazy 

Mountains, Montana..................................................................................................................10
iv

Tables

1. Definition and characterization of data fields included in appendix 1 (status and 

treatment of samples)..................................................................................................................3

2. Definition and characterization of data fields included in appendix 2 (geochemical 

and petrographic data)................................................................................................................4

Abbreviations

Al2O3 aluminum oxide

Am americium

Ba barium

CaO calcium oxide

Cd cadmium

Ce cerium

FeO ferrous iron

K potassium

K2O potassium oxide

La lanthanum

MgO magnesium oxide

MnO manganese oxide

Na2O sodium oxide

Nb niobium

Nd neodymium

P2O5 phosphorus pentoxide

Rb rubidium

SiO2 silicon dioxide

Sr strontium

TiO2 titanium dioxide

Y yttrium

Zr zirconium

ppm parts per million
Geochemical and Petrographic Data for Intrusions 

Peripheral to the Big Timber Stock, Crazy Mountains, 

Montana

By Edward A. du Bray, Anna B. Wilson, and Bradley S. Van Gosen

Introduction

In the Crazy Mountains of south-central Montana, the 

Paleocene Fort Union Formation hosts a compositionally 

diverse array of Eocene plugs, dikes, and sills arrayed around 

the Eocene Big Timber stock; all of these rocks are part of 

the Crazy Mountains magmatic system. As documented 

by du Bray and Harlan (1996) and Dudas (1990, 1991), 

Eocene igneous rocks in the Crazy Mountains include silicaundersaturated, strongly alkaline intrusive rocks and silicasaturated, alkaline to subalkaline intrusive rocks that are 

approximately coeval. Most of the strongly alkaline rocks are 

in the northern part of the Crazy Mountains and form sills, 

laccoliths, small stocks, and dikes. The strongly alkaline plug 

at Ibex Mountain (fig. 1) is located significantly farther south 

than other strongly alkaline rocks associated with the Crazy 

Mountains magmatic system. Compositionally less exotic 

alkaline to subalkaline rocks, including constituents of the 

Big Timber stock (du Bray and Harlan, 1996), form stocks 

and associated dike swarms that are mostly restricted to the 

southern part of the range (fig. 1). Other Eocene intrusions, 

including outcrops at Gobblers Knob, Raspberry Butte, 

and other nearby conformable intrusions form a set of sills 

that are satellitic to the Big Timber stock and constitute 

a third, relatively unstudied set of intrusions in the Crazy 

Mountains (fig. 1).

The distribution of intrusive rocks in the Crazy Mountains is portrayed on various geologic maps of the area, 

including those by Wolff (1938), Simms (1966), Tappe (1966), 

Starmer (1972), and du Bray and others (1993). The petrology 

and petrogenesis of the strongly alkaline rocks in particular 

were well documented by Dudas (1990, 1991). Subsequently, 

du Bray and Harlan (1996) conducted a detailed investigation 

concerning the petrogenesis of the Big Timber stock, and 

du Bray and others (2006) characterized the radial dike swarm 

associated with the Big Timber stock. None of these studies, 

however, have documented and synthesized the geochemistry and petrography of the sills that are peripheral to the Big 

Timber stock.

The purpose of this report is (1) to present available geochemical and petrographic data for several dozen igneous rock 

samples, which represent sills and plugs peripheral to the 

Big Timber stock, and (2) to provide a basic interpretive 

synthesis of these data. These samples were collected in 

1992 during geologic mapping of the Big Timber stock by 

du Bray and others (1993). During field studies, 28 outcrop 

samples were collected for subsequent laboratory analysis. 

Petrographic data were acquired for 25 of these samples, and 

geochemical analyses were acquired for 26 of these samples. 

Five samples of strongly alkaline rock, four from the plug at 

Ibex Mountain and one from a small plug exposed along the 

Shields River northwest of the Big Timber stock, are petrographically and geochemically distinct relative to all other 

samples described herein.

Analytical Methods

Petrographic characteristics of samples were determined 

using a standard petrographic microscope. All whole-rock 

major oxide chemical abundances were determined by 

wavelength-dispersive X-ray fluorescence spectrometry, using 

methods described by Taggart and others (1987), in analytical 

laboratories of the U.S. Geological Survey, Denver, Colorado. 

All iron abundances were converted to ferrous iron, and each 

major oxide analysis was recalculated to 100 percent on a 

volatile-free basis. Trace-element abundances were determined 

by energy-dispersive X-ray fluorescence spectroscopy (Elsass 

and du Bray, 1982; Yager and Quick, 1992) using 109Cd and 241Am radio-isotope excitation sources. All censored values 

were replaced by blank cells; for lower limits of determination, see Elsass and du Bray (1982).

Data Fields

Data were compiled using Microsoft Excel; they are 

presented here in two appendixes, found both at the back of 

this report and as Excel files (Appendix1.xlsx and Appendix2.xlsx) linked from this document and from http://dx.doi.

org/10.3133/ds895. The files can be accessed using software 
2 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Figure 1. Regional geologic setting for the Crazy Mountains, Montana, showing the Big Timber stock, associated radial dikes, 

and peripheral sills and plugs. Collection sites for all samples described herein are labelled; complete sample numbers for 

samples ending in “V” or “W” include the suffix “92.” Geologic features modified from Roberts (1972). (Click here to open fullsize, high-resolution image.)

0 5

0 5 10 15 20 KILOMETERS

Base from Army Map Service, 1958, White

Sulphur Springs and Bozeman, Mont., 1:250,000

Transverse Mercator projection, Zone 12

North American Datum of 1927 (NAD 27)

023W, 024W, 025W,

026W, 027W, 028W

022W

D

DD

D

DD

DDD

DDDD

D

D

D 021W

D 004W, 005W,

 006W, 007W, 008W

D

Gobblers

 Knob

 sill

 Ibex

Mountain

 plug

Loco

Mountain

stock

Porcupine

Butte

sill

Big

Timber

stock

Raspberry

 Butte

 sill

 Billie

Butte

 plug

Alkaline to subalkaline

dike

Alkaline to subalkaline

stock or sill

Strongly alkaline plug 

or sill

D Sample site and

sample number

MONTANA

Map area

Shields River

EXPLANATION

011W, 012W 013W, 014W, 015W

003V

002W 

202338, 001V 

018W 

017W

016W 

010W 

003W 

009W 

020W

019W

0014V

010W

001W

10°45’ 110°30’ 110°15’ 46°15’ 110°00’

46°00’

10 MILES
Geochemical Characteristics 3

compatible with the .xlsx file format. Sample characterization, geochemical, and petrographic data are presented in 

columns or sets of related columns. The contents of appendix 

1 constitute basic sample information, including sample location, sample treatment, and lithologic characterization for each 

sample; see table 1 for data field definitions. Appendix 2 contains geochemical and petrographic observations; see table 2 

for data field definitions. Geochemical data in some worksheet 

cells might appear to be more precise than displayed values, 

but the implied precision is a misleading artifact of computational processes used to create data-cell contents (for instance, 

recalculation to 100-percent volatile free). Blank cells in the 

appendix worksheets indicate either null values or that no 

data are available. In appendix 2, some blank cells reflect 

abundances that were reported as “less than the lower limit of 

determination for the analytical method used;” these values 

were replaced by blank cells to enable statistical analysis of 

the uncensored data.

Geochemical Characteristics

Major Oxide Data

As is true of the Big Timber stock and its associated 

radial dike swarm (du Bray and Harlan, 1996; du Bray and 

others, 2006), major oxide characteristics of the sill-forming 

rocks peripheral to the Big Timber stock are consistent with a 

subduction-related petrogenesis. Compositions of three samples of the Ibex Mountain plug and one sample from a plug 

along the Shields River are quite distinct and are described 

separately. Relative to standard metrics (in cited sources), the 

sill-forming rocks are metaluminous (fig. 2) (Shand, 1951), 

magnesian (fig. 3) (Frost and others, 2001), and have compositions (fig. 4) that straddle the alkaline-subalkaline dividing line 

of Irvine and Baragar (1971). In terms of the balance between 

abundances of CaO, Na2O, and K2O in these rocks, the sill 

compositions vary significantly across the entire calcic, calcalkalic, alkali-calcic, and alkalic spectrum (fig. 5), likely a 

misleading consequence of post-magmatic alteration and alkali 

mobility experienced by these rocks. Abundances of SiO2 in 

the sill-forming rocks range nearly continuously from about 46 

to 63 weight percent (fig. 4), and their compositions range from 

basalt to dacite and their alkaline analogs. Concentrations of 

TiO2 and MgO vary considerably at lower SiO2 abundances 

but scatter less and decrease to lower values at higher SiO2

contents (fig. 6). Concentrations of FeO* (total iron, expressed 

in the ferrous state), MnO, and CaO (fig. 6) decrease in a 

linear fashion with increasing SiO2. Abundances of Na2O and 

P2O5 vary widely and yield no consistent variation relative to 

SiO2 content. Abundances of Al2O3 and K2O increase broadly 

with increasing SiO2 content; the K2O data array is dominated 

by transitional high-potassium to shoshonitic compositions 

(Gill, 1981). These trends and compositional ranges are 

similar to and overlap those of the Big Timber stock and its 

associated radial dike swarm (du Bray and Harlan, 1996; 

du Bray and others, 2006).

Primary mafic magmas that assimilate crustal contaminants predictably evolve to more silicic compositions characterized by progressively lower P2

O5/K2O because crustal 

materials generally have P2O5/K2O less than 0.1 (Farmer 

and others, 2002). Among the sill-forming rocks, P2O5/K2O 

decreases with increasing SiO2 content and increases with 

increasing MgO content, which suggests that compositions 

of the magmas represented by these rocks evolved through 

variable contamination of primary mafic partial melts by 

crustally derived inputs. Similarly, Cousens and others (2008) 

suggested that decreasing CaO/Al2O3 with increasing SiO2

principally reflects crustal contamination. Among the sillforming rocks, CaO/Al2

O3 decreases with increasing SiO2, 

which corroborates the influence of progressive crustal contamination in the development of these rocks.

Table 1. Definition and characterization of data fields included in appendix 1 (status and treatment of samples).

FIELD_NAME FIELD_DESCRIPTION

Field_ID Field-assigned sample identifier; Field_ID entries may link data in individual rows to the contents of particular rows in 

the other appendix or to the National Geochemical Database.

Longitude In decimal degrees, relative to the North American Datum of 1927. Longitude is reported as a negative value (western 

hemisphere).

Latitude In decimal degrees, relative to the North American Datum of 1927. Latitude is reported as a positive value (northern 

hemisphere).

Chem “X” indicates chemical analysis for sample obtained (see appendix 2).

TS “X” indicates thin section of sample prepared and examined using a petrographic microscope (see appendix 2).

Locality_name Prominent geographic feature proximal to sample site(s).

Lithology Sample composition according to the classification scheme of Le Maitre (2002).

Igneous_form Form (sill, dike, or plug) of the igneous rock represented by each sample.

Alteration Geochemical characteristics indicative of alteration; high LOI (loss on ignition) equates to values >3 weight percent, and 

low K2

O equates to values <0.5 weight percent.
4 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data).

FIELD_NAME FIELD_DESCRIPTION

Field_ID Field-assigned sample identifier; Field_ID entries may link data in individual rows to the contents of particular rows in the other appendix or to the National Geochemical Database.

Locality_name Prominent geographic feature proximal to sample site(s).

SiO2_pct Silicon, as silicon dioxide, in weight percent; based on major oxide data recalculated to 100 percent on a 

volatile-free basis.

TiO2_pct Titanium, as titanium dioxide, in weight percent; based on major oxide data recalculated to 100 percent on a 

volatile-free basis.

Al2O3_pct Aluminum, as aluminum trioxide, in weight percent; based on major oxide data recalculated to 100 percent 

on a volatile-free basis.

FeO*_pct Total iron, as ferrous oxide, in weight percent; based on major oxide data recalculated to 100 percent on a 

volatile-free basis.

MnO_pct Manganese, as manganese oxide, in weight percent; based on major oxide data recalculated to 100 percent 

on a volatile-free basis.

MgO_pct Magnesium, as magnesium oxide, in weight percent; based on major oxide data recalculated to 100 percent 

on a volatile-free basis.

CaO_pct Calcium, as calcium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a 

volatile-free basis.

Na2O_pct Sodium, as sodium oxide, in weight percent; based on major oxide data recalculated to 100 percent on a 

volatile-free basis.

K2O_pct Potassium, as potassium oxide, in weight percent; based on major oxide data recalculated to 100 percent on 

a volatile-free basis.

P2O5_pct Phosphorus, as phosphorus pentoxide, in weight percent; based on major oxide data recalculated to 

100 percent on a volatile-free basis.

LOI_pct Volatile content lost on ignition, in weight percent.

Total_I_pct Initial, pre-recalculation sum of oxide abundances, in weight percent.

Ba_ppm Barium, in parts per million.

Rb_ppm Rubidium, in parts per million.

Sr_ppm Strontium, in parts per million.

Y_ppm Yttrium, in parts per million.

Zr_ppm Zirconium, in parts per million.

Nb_ppm Niobium, in parts per million.

Th_ppm Thorium, in parts per million.

Ga_ppm Gallium, in parts per million.

La_ppm Lanthanum, in parts per million.

Ce_ppm Cerium, in parts per million.

Nd_ppm Neodymium, in parts per million.

Cu_ppm Copper, in parts per million.

Pb_ppm Lead, in parts per million.

Zn_ppm Zinc, in parts per million.

Sn_ppm Tin, in parts per million.

W_ppm Tungsten, in parts per million.

As_ppm Arsenic, in parts per million.

Sb_ppm Antimony, in parts per million.

Abd_Pl_phenos Modal abundance of plagioclase phenocrysts relative to the whole rock, in volume percent.

Abd_GrnAmph_phenos Modal abundance of green amphibole (magnesio-hornblende) phenocrysts relative to the whole rock, in 

volume percent.
Geochemical Characteristics 5

Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data).—Continued

FIELD_NAME FIELD_DESCRIPTION

Abd_BrnAmph_phenos Modal abundance of brown amphibole (magnesio-hastingsite) phenocrysts relative to the whole rock, in 

volume percent.

Abd_Bt_phenos Modal abundance of biotite phenocrysts relative to the whole rock, in volume percent—TR, trace amounts 

(<0.5 volume percent).

Abd_Cpx_phenos Modal abundance of clinopyroxene phenocrysts relative to the whole rock, in volume percent.

Abd_Ol_phenos Modal abundance of olivine phenocrysts relative to the whole rock, in volume percent—TR, trace amounts 

(<0.5 volume percent).

Abd_Opq Modal abundance of opaque iron-titanium oxide minerals relative to the whole rock, in volume percent.

TotXtls Microscope-based estimate of total phenocryst content relative to the whole rock, in volume percent.

ClrIndx Microscope-based estimate of color index (sum of the abundances of hornblende, biotite, pyroxene, olivine, 

and opaque iron-titanium oxide minerals), in volume percent.

AgsPl Microscope-based estimate of average grain size of plagioclase phenocrysts, in millimeters.

AgsGrnAmph Microscope-based estimate of average grain size of green amphibole (magnesio-hornblende) phenocrysts, 

in millimeters.

AgsBrnAmph Microscope-based estimate of average grain size of brown amphibole (magnesio-hastingsite) phenocrysts, 

in millimeters.

AgsBt Microscope-based estimate of average grain size of biotite phenocrysts, in millimeters.

AgsCpx Microscope-based estimate of average grain size of clinopyroxene phenocrysts, in millimeters.

AgsOl Microscope-based estimate of average grain size of olivine phenocrysts, in millimeters.

AgsOpq Microscope-based estimate of average grain size of opaque iron-titanium oxide phenocrysts, in millimeters.

MgsPl Microscope-based estimate of maximum grain size (length) of largest plagioclase phenocryst, in millimeters.

MgsGrnAmph Microscope-based estimate of maximum grain size (length) of largest green amphibole (magnesiohornblende) phenocryst, in millimeters.

MgsBrnAmph Microscope-based estimate of maximum grain size (length) of largest brown amphibole (magnesiohastingsite) phenocryst, in millimeters.

MgsBt Microscope-based estimate of maximum grain size (length) of largest biotite phenocryst, in millimeters.

MgsCpx Microscope-based estimate of maximum grain size (length) of largest clinopyroxene phenocryst, 

in millimeters.

MgsOl Microscope-based estimate of maximum grain size (length) of largest olivine phenocryst, in millimeters.

MgsOpq Microscope-based estimate of maximum grain size (length) of largest opaque iron-titanium oxide phenocryst, in millimeters.

Texture Characteristic petrographic textures as determined by microscopic observation—Aph, aphanitic; Hy, 

hyalophitic; Hc, holocrystalline; I, intersertal; E, equigranular; P, porphyritic; S, seriate; T, trachytic.

Access_Mnrls Accessory minerals identified by microscopic observation; listed in order of decreasing abundance—

Ap, apatite; Ttn, titanite.

XlPl Microscope-based estimate of crystallinity of plagioclase phenocrysts—A, anhedral; S, subhedral; 

E, euhedral.

XlGrnAmph Microscope-based estimate of crystallinity of green amphibole (magnesio-hornblende) phenocrysts—

A, anhedral; S, subhedral; E, euhedral.

XlBrnAmph Microscope-based estimate of crystallinity of brown amphibole (magnesio-hastingsite) phenocrysts—

A, anhedral; S, subhedral; E, euhedral.

XlBt Microscope-based estimate of crystallinity of biotite phenocrysts—A, anhedral; S, subhedral; E, euhedral.

XlCpx Microscope-based estimate of crystallinity of clinopyroxene phenocrysts—A, anhedral; S, subhedral; 

E, euhedral. If more than one crystallinity type is present, the dominant form is listed first.

XlOl Microscope-based estimate of crystallinity of olivine phenocrysts—A, anhedral; S, subhedral; E, euhedral.

XlOpq Microscope-based estimate of crystallinity of opaque iron-titanium oxide phenocrysts—A, anhedral; 

S, subhedral; E, euhedral.
6 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Table 2. Definition and characterization of data fields included in appendix 2 (geochemical and petrographic data).—Continued

FIELD_NAME FIELD_DESCRIPTION

Petrog_Com Groundmass characteristics and otherwise noteworthy features. Groundmass minerals include plagioclase 

(Pl), hornblende (Hbl), clinopyroxene (Cpx), biotite (Bt), quartz (Qtz), and opaque iron-titanium minerals 

(Opq). The presence of secondary, alteration minerals, including chlorite (Chl), calcite (Cc), or epidote 

(Ep), is noted. [%, percent; mm, millimeters]

HblClr Pleochroic colors of hornblende phenocrysts, if present.

AltExtnt Microscope-based estimate of the extent of alteration—1 indicates a completely fresh sample, and 

5 indicates a completely altered sample in which primary textures and minerals are not identifiable. 

Intermediate values of 2 through 4 identify progressively more altered samples.

Figure 2. Variation diagram showing relative alumina and alkali saturation of igneous rocks 

peripheral to the Big Timber stock, Crazy Mountains, Montana, as a function of molar major-oxide 

compositions.

0.25

0.50

0.75

1.00

1.25

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Al2O3/(Na2O + K2O + CaO), molar

Metaluminous Peraluminous

Peralkaline

Sill-forming sample

Plug-forming sample

EXPLANATION

(Na O + K 2 O)/Al

2 O2 3, molar
Geochemical Characteristics 7

Figure 4. Total alkali-silica variation diagram showing compositions of igneous rocks peripheral to 

the Big Timber stock, Crazy Mountains, Montana. Field boundaries from Le Maitre (2002). Alkalinesubalkaline dividing line from Irvine and Baragar (1971).

0

5

10

15

40 45 50 55 60 65 70 75

Na O+K 2 O, in weight percent

2

Trachydacite

Andesite

Dacite

Trachyandesite

Basaltic

trachyandesite

Trachybasalt

Basalt Basaltic

andesite

Phonotephrite Alkaline

Subalkaline

SiO2, in weight percent

Sill-forming sample

Plug-forming sample

EXPLANATION

Figure 3. Variation diagram showing FeO/(FeO+MgO) values for igneous rocks peripheral to the 

Big Timber stock, Crazy Mountains, Montana, relative to boundaries between ferroan and magnesian 

rocks. Ferroan-magnesian boundary from Frost and others (2001).

0.5

0.6

0.7

0.8

45 47 49 51 53 55 57 59 61 63 65

SiO2, in weight percent

Ferroan

Magnesian

Sill-forming sample

Plug-forming sample

EXPLANATION

FeO*/(FeO*+MgO)
8 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Many geochemical features distinguish the compositions

of the samples of the Ibex Mountain and plugs along the 

Shields River (fig. 1) from those of the sill-forming rocks. 

First, all four of these representative samples are composed 

of phonotephrite and have strongly alkaline compositions 

(fig. 4) relative to the alkaline-subalkaline dividing line of

Irvine and Baragar (1971). Three of these four samples have 

agpaitic indices greater than 1 (fig. 2) and are therefore 

peralkaline; the fourth contains aegirine, which is likewise 

diagnostic of peralkaline magmas. The silica content of these 

rocks, about 48 to 49 weight percent, is significantly lower 

than that of most of the sill-forming rocks. Both the Na2O and 

K2O contents of these rocks are elevated, given their low silica 

contents, but they are especially sodic, with Na2O contents that 

range from about 5 to 8 weight percent (fig. 6). Like all other 

rocks in the Crazy Mountains, the strongly alkaline rocks are 

magnesian (fig. 3). Relative proportions of Na2O, K2O, and 

CaO in these rocks are consistent with alkalic to strongly 

alkali-calcic compositions (fig. 5). Relative to compositional 

trends depicted by all other alkaline to subalkaline rocks in the 

Crazy Mountains (fig. 6), the strongly alkaline rocks have low 

TiO2

 and CaO abundances, high Na2O and K2O abundances, 

and remarkably high P2O5 abundances (fig. 6). Their Al2O3, 

FeO, MnO, and MgO abundances are approximately on 

trend with those of other alkaline to subalkaline rocks in the 

Crazy Mountains.

Trace-Element Data

Several aspects of trace-element data available for sillforming alkaline to subalkaline rocks of the Crazy Mountains 

are noteworthy. In particular, abundances of Ba (range, about 

600–4,300 parts per million [ppm]; average, about 2,200) 

and Sr (range, about 400–1,400 ppm; average, about 900) in 

these rocks are especially elevated relative to the concentrations of those elements in most igneous rocks (Turekian and 

Wedepohl, 1961). Rubidium (Rb) abundances in these rocks 

are relatively low, resulting in very low Rb/Sr values (average, 

0.06). Abundances of Y, Zr, Nb, La, Ce, and Nd in these rocks 

are similar to those of other convergent-margin, broadly calcalkaline igneous rocks, such as those in the Andean, Kamchatka, and Central American volcanic arcs (Max Plank Institut für Chemie, 2010). Among the sill-forming rocks, Rb, Zr, 

La, and Ce abundances increase systematically with increasing 

silica content; Y abundances decrease; and Ba, Sr, Nb, and Nd 

abundances are uncorrelated with varying silica content. Most 

of the sill-forming rocks have relative abundances of Rb and 

Y+Nb that are consistent with a genesis in a volcanic arc 

setting (fig. 7).

Most continental magmatic arc rocks have Ba/Nb greater 

than 15 (Gill, 1981). The sill-forming rocks have Ba/Nb 

values that average about 126, range upward to almost 240, 

and do not vary systematically with respect to silica content. 

Figure 5. Variation diagram showing Na2O+K2O-CaO values versus SiO2 content among samples of 

igneous rocks peripheral to the Big Timber stock, Crazy Mountains, Montana. Boundaries between 

various rock series from Frost and others (2001).

-9.0

-7.0

-5.0

-3.0

-1.0

1.0

3.0

5.0

7.0

47 49 51 53 55 57 59 61 63 65

Calcic

Calc-alkalic

Alkali-calcic

Alkalic

Na O+K 2 O-CaO, in weight percent

2

SiO2, in weight percent

Sill-forming sample

Plug-forming sample

EXPLANATION
Geochemical Characteristics 9

Figure 6. Variation diagrams showing abundances of major oxides in igneous rocks peripheral to the Big Timber stock, 

Crazy Mountains, Montana. All abundances in weight percent, except zirconium (in parts per million). Field boundaries on 

K2

O versus SiO2 diagram from Le Maitre (2002); high-K–shoshonitic dividing line from Ewart (1982).

TiO2

FeO*

MnO

MgO

CaO

SiO2, in weight percent

Abundance of oxide, in weight percent

Abundance of oxide, in weight percent

Abundance of zirconium,

in parts per million

SiO2, in weight percent

Na20

K2

O

P2

O5

1.0

1.2

1.4

0.09

0.11

0.13

0.15

0.17

0.19

0.21

13

14

15

16

17

18

6

7

8

9

10

11

1

2

3

4

5

6

7

8

45 47 49 51 53 55 57 59 61 63 65 45 47 49 51 53 55 57 59 61 63 65

5

7

9

11

13

2

3

4

5

6

7

8

1

2

3

4

5

Low-K

Medium-K

High-K

Shoshonitic

0.50

0.75

1.00

150

200

250

Zr

100

Sill-forming sample

Plug-forming sample

EXPLANATION

1.6

Al203
10 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Elevated Ba/Nb values have been associated with those mantle 

wedge magmas that derived subducted slab components 

through dehydration of the subducted-slab and attendant fluid 

flux–induced partial melting (Hawkesworth and others, 

1995; Pearce and Peate, 1995; Cousens and others, 2008; 

Schmidt and others, 2008). Accordingly, highly elevated 

Ba/Nb ratios and noteworthy large-ion lithophile element 

(Ba and Sr) enrichments suggest significant involvement of a 

subducted-slab-derived fluid component in the petrogenesis of 

the magmas represented by the sill-forming rocks.

Trace-element characteristics of the strongly alkaline 

rocks are highly distinctive. Although Ba and Sr abundances 

in the sill-forming rocks are elevated (averaging about 3,200 

and 2,700 ppm, respectively), those for the strongly alkaline 

rocks are even greater. Rubidium abundances in the strongly 

alkaline rocks are higher, averaging about 71 ppm; however, 

their average Rb/Sr ratio is even lower (0.03) than that for 

the sill-forming rocks. Similarly, Y, Zr, and Nb abundances 

of the strongly alkaline rocks are significantly higher than 

those of the sill-forming rocks. The average Zr content of 

the strongly alkaline rocks (221 ppm) is lower than might 

be expected given their alkalinity and experimental work by 

Watson (1979), which demonstrated that the alkaline magmas, such as those represented by these rocks, can contain 

significantly greater zirconium concentrations before zircon 

saturation is achieved and zirconium concentrations become 

buffered. Finally, the light rare earth element (La, Ce, and 

Nd) abundances of the strongly alkaline rocks are strikingly 

elevated relative to those characteristic of other igneous rocks 

(Turekian and Wedepohl, 1961). These trace-element characteristics suggest that the petrogenetic history of these alkaline 

rocks was quite different from that responsible for magmas 

represented by the sill-forming rocks. A significant aspect of 

these distinctive characteristics is underscored by the fact that 

relative abundances of Rb and Y+Nb for the strongly alkaline 

rocks coincide with the within-plate field on the trace-element, 

tectonic setting–discrimination variation diagram (fig. 7), 

whereas those for the sill-forming rocks are equivalent to 

volcanic arc compositions.

Petrographic Characteristics

Most samples of the sill-forming rocks described herein 

have similar petrographic characteristics; in contrast, samples 

of the strongly alkaline plug at Ibex Mountain are petrographically distinct and thus are described separately. Many of the 

sill-forming rocks contain either green or brown amphibole. 

Electron microprobe analyses of green and brown amphibole 

contained in the nearby Big Timber stock indicate that they are 

composed of magnesio-hornblende and magnesio-hastingsite, 

respectively (du Bray and Harlan, 1996). Optical features of 

the green amphibole in sill and Big Timber stock samples are 

indistinguishable, which suggests that the green amphibole 

in the sills is also magnesio-hornblende. Similarly, brown 

amphibole in the sills and the stock are optically indistinguishable, which suggests that brown amphibole in the sills is 

magnesio-hastingsite. The sill rocks are variably porphyritic. 

Phenocryst abundances range from 0 to 65 percent, averaging 

about 20 percent. Color index ranges from 1 to 35 percent, 

averaging about 13 percent. Fine- to medium-grained phenocryst assemblages include combinations of plagioclase, 

magnesio-hornblende, magnesio-hastingsite, clinopyroxene, 

and rare biotite in a fine-grained groundmass composed of 

combinations of plagioclase, magnesio-hornblende, magnesiohastingsite, clinopyroxene, opaque iron-titanium oxides, and 

variably devitrified glass. The groundmass minerals form 

intersertal intergrowths in most samples, although in several 

samples the groundmass consists of intergranular intergrowths.

Among samples of dikes associated with the Big Timber

stock, the composition of the mafic silicate minerals is 

strongly correlated with whole-rock composition (du Bray 

and others, 2006); basaltic trachyandesite dikes are dominated 

by clinopyroxene, low-silica trachyandesite dikes contain 

clinopyroxene and magnesio-hastingsite, high-silica trachyandesite to low-silica trachydacite dikes contain magnesiohastingsite, and high-silica trachydacite to rhyolite dikes 

contain biotite and magnesio-hornblende. The composition of 

the mafic silicate minerals contained in the sill-forming rocks 

correlate similarly, but less systematically, with whole-rock 

composition. Accessory minerals in the sill-forming rocks are 

Figure 7. Trace-element, tectonic setting–

discrimination variation diagram showing the 

composition of igneous rocks peripheral to the Big 

Timber stock, Crazy Mountains, Montana. Tectonic 

setting–composition boundaries from Pearce and 

others (1984).

10

100

10 100

Rb, parts per million

Y + Nb, in parts per million

Volcanic

 arc

Within

 plate

Sill-forming sample

Plug-forming sample

EXPLANATION
References Cited 11

rare, though apatite was identified in several samples. The 

groundmass of many samples contains secondary calcite and 

(or) chlorite related to post magmatic alteration of many of 

these rocks. Similarly, many primary phenocrysts are variably 

altered and have been replaced by calcite and clay or sericite.

The plug at Ibex Mountain is composed of rock that 

ranges from holocrystalline and equigranular (sample 202338) 

to porphyritic with a hyalophitic groundmass (sample 002W92). 

Pale green clinopyroxene is the dominant mineral in both of 

these samples; other mafic silicate minerals include tan to 

distinctly red-brown biotite and trace amounts of subhedral 

olivine. Clinopyroxene in sample 202338 is overgrown by 

distinctive emerald-green aegirine rims, and in both Ibex 

Mountain samples is distinguished by a well-developed sieve 

texture. These rocks lack quartz and feldspar but, befitting 

their strongly alkaline character, they contain nepheline, which 

forms anhedral, interstitial grains in sample 202338 and euhedral phenocrysts in sample 002W92. Some nepheline is partly 

replaced by and (or) overgrown by acicular sprays of zeolite 

minerals. Sample 002W92 is further distinguished by the presence of cancrinite, which forms small discrete grains and overgrowths on nepheline crystals. The occurrence of cancrinite 

in strongly alkaline rocks, particularly in the northern Crazy 

Mountains, was highlighted by the detailed investigations of 

Simms (1966). Apatite is a characteristic accessory constituent 

of the Ibex Mountain plug.

Synthesis

The most significant finding derived from data presented 

herein is that the geochemical and petrographic characteristics 

of the sill-forming rocks are indistinguishable from those of 

the Big Timber stock and its radial dike swarm. Consequently, 

the central stock, associated radial dikes, and surrounding 

sills seem to be part of a single, coeval magmatic episode in 

the Crazy Mountains. By analogy, other still unsampled and 

undocumented sill-forming intrusions peripheral to and south 

and east of the Big Timber stock are probably also composed 

of rock related to magmatism responsible for formation of 

the Big Timber stock. In contrast, the small amount of data 

for the plug-forming rocks at Ibex Mountain and along the 

Shields River indicate that the geochemistry and petrography 

of these rocks are significantly different from those of the 

sill-forming rocks but remarkably similar to those of other 

strongly alkaline igneous rocks exposed north and west of the 

Big Timber stock.

Characteristics of the sill-forming intrusions are in accord 

with their petrogenesis in a subduction-related setting, as is 

also true for the Big Timber stock and its radial dike swarm 

(du Bray and Harlan, 1996). Unusual trace-element abundances, especially elevated concentrations of Ba and Sr, are 

consistent with significant crustal inputs through assimilation 

and (or) magmatism involving noteworthy inputs derived from 

fluids released during devolatilization of the downgoing, subducted slab. Finally, the geospatial and temporal coincidence 

of strongly alkaline and alkaline to subalkaline 

magmas in the Crazy Mountains remains something of a 

petrologic paradox.

Acknowledgments

Data compilation undertaken for this study was conducted as part of the mineral resource assessment of Custer 

and Gallatin National Forests funded by the U.S. Geological 

Survey Mineral Resources Program. Constructive reviews by 

Ryan D. Taylor and Matthew Granitto are much appreciated 

and helped clarify data presentation.

References Cited

Cousens, B.L., Prytulak, Julie, Henry, C.D., Alcazar, Al, and 

Brownrigg, Tim, 2008, Geology, geochronology, and geochemistry of the Miocene-Pliocene ancestral Cascades arc, 

northern Sierra Nevada, California and Nevada—The roles 

of the upper mantle, subducting slab, and the Sierra Nevada 

lithosphere: Geosphere, v. 4, p. 814–828.

du Bray, E.A., Elliott, J.E., Wilson, A.B., Van Gosen, B.S., and 

Rosenberg, L.A., 1993, Geologic map of the Big Timber 

stock and vicinity, southern Crazy Mountains, Sweet Grass 

and Park Counties, south-central Montana: U.S. Geological Survey Miscellaneous Field Investigations Series Map 

MF-2253, scale 1:24,000.

du Bray, E.A., and Harlan, S.S., 1996, The Eocene Big Timber 

stock, south-central Montana—Development of extensive 

compositional variation in an arc-related intrusion by 

side-wall crystallization and cumulate glomerocryst 

remixing: Geological Society of America Bulletin, v. 108, 

p. 1404–1424.

du Bray, E.A., Harlan, S.S., and Wilson, A.B., 2006, Petrology of the Crazy Mountains dike swarm and geochronology 

of associated sills, south-central Montana: U.S. Geological 

Survey Professional Paper 1715, 21 p.

Dudas, F.O., 1990, Petrogenesis and mantle source of igneous 

rocks in the Crazy Mountains, Montana: University Park, 

Pennsylvania State University, Ph.D. thesis, 442 p.

Dudas, F.O., 1991, Geochemistry of igneous rocks from the 

Crazy Mountains, Montana, and tectonic models for the 

Montana alkalic province: Journal of Geophysical Research, 

v. 96, p. 13261–13277.

Elsass, Françoise, and du Bray, E.A., 1982, Energy-dispersive 

X-ray fluorescence spectrometry with the Kevex 7000 

system: Saudi Arabia, Deputy Ministry of Mineral 

Resources Open-File Report USGS-OF-02-52, 53 p.
12 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Ewart, Anthony, 1982, The mineralogy and petrology of 

Tertiary—Recent orogenic volcanic rocks with special 

reference to the andesitic-basaltic compositional range, in 

Thorpe, R.S., ed., Andesites: New York, John Wiley and 

Sons, p. 25–87.

Farmer, G.L., Glazner, A.F., and Manley, C.R., 2002, Did 

lithospheric delamination trigger late Cenozoic potassic 

volcanism in the southern Sierra Nevada, California?: 

Geological Society of America Bulletin, v. 114, p. 754–768.

Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, 

D.J., and Frost, C.D., 2001, A geochemical classification for 

granitic rocks: Journal of Petrology, v. 42, p. 2033–2048.

Gill, J.B., 1981, Orogenic andesites and plate tectonics: New 

York, Springer-Verlag, 390 p.

Hawkesworth, Chris; Turner, Simon; Gallagher, Kerry; 

Hunter, Arlene; Bradshaw, Tim; and Rogers, Nick, 1995, 

Calc-alkaline magmatism, lithospheric thinning and 

extension in the Basin and Range: Journal of Geophysical 

Research, v. 100, p. 10271–10286.

Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian 

Journal of Earth Sciences, v. 8, p. 523–548.

Le Maitre, R.W., 2002, Igneous rocks—A classification and 

glossary of terms (2d ed.): Cambridge, United Kingdom, 

Cambridge University Press, 236 p.

Max Plank Institut für Chemie, 2010, Geochemistry of rocks 

of the oceans and continents (GEOROC) [Query by geological setting—convergent margins]: Mainz, Germany, 

Max Plank Institut für Chemie, accessed June 10, 2014, at 

http://georoc.mpch-mainz.gwdg.de/georoc/.

Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace 

element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, 

p. 956–983.

Pearce, J.A., and Peate, D.W., 1995, Tectonic implications of 

the composition of volcanic arc magmas: Annual Review of 

Earth and Planetary Sciences, v. 23, p. 251–285.

Roberts, A.E., 1972, Cretaceous and early Tertiary depositional and tectonic history of the Livingston area, southwestern Montana: U.S. Geological Survey Professional 

Paper 526-C, 120 p.

Schmidt, M.E., Grunder, A.L., and Rowe, M.C., 2008, Segmentation of the Cascade arc as indicated by Sr and Nd 

isotopic variation among diverse primitive basalts: Earth 

and Planetary Science Letters, v. 266, p. 166–181.

Shand, S.J., 1951, Eruptive rocks: New York, John Wiley, 

488 p.

Simms, F.E., 1966, The igneous petrology, geochemistry, 

and structural geology of part of the northwestern Crazy 

Mountains, Montana: Cincinnati, Ohio, University of 

Cincinnati, Ph.D. thesis, 339 p.

Starmer, R.J., 1972, The distribution and geochemistry of the 

Big Timber dike swarm, Crazy Mountains, Montana: Cincinnati, Ohio, University of Cincinnati, Ph.D. thesis, 90 p.

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, 

A.J., and Stewart, K.C., 1987, Analysis of geologic materials by X-ray fluorescence spectrometry, Chapter E of 

Baedecker, P.A., ed., Methods for geochemical analysis: 

U.S. Geological Survey Bulletin 1770, p. E1–E19.

Tappe, John, 1966, The chemistry, petrology, and structure 

of the Big Timber igneous complex, Crazy Mountains, 

Montana: Cincinnati, Ohio, University of Cincinnati, 

Ph.D. thesis, 134 p.

Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of 

the elements in some major units of the Earth’s crust: 

Geological Society of America Bulletin, v. 72, p. 175–192.

Watson, E.B., 1979, Zircon saturation in felsic liquids—

Experimental results and applications to trace element 

geochemistry: Contributions to Mineralogy and Petrology, 

v. 70, p. 407–419.

Wolff, J.E., 1938, Igneous rocks of the Crazy Mountains, 

Montana: Geological Society of America Bulletin, v. 49, 

p. 1569–1626.

Yager, D.B., and Quick, J.E., 1992, SUPERXAP manual: U.S. 

Geological Survey Open-File Report 92-13, 45 p.

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Central Mineral and 

Environmental Resources Science Center

Box 25046, Mail Stop 973

Denver, CO 80225

(303) 236-1562

Or visit the Central Mineral and Environmental Resources Science 

Center Web site at: 

http://minerals.cr.usgs.gov/
Appendixes
14 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Appendix 1. Status and treatment of samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.

[See table 1 for an explanation of data fields. Chem, chemical analysis; TS, thin section; LOI, loss on ignition; K2O, potassium oxide]

Field_ID Longitude Latitude Chem TS Locality_name Lithology Ignous_form Alteration

202338 –110.448 45.993 X X Ibex Mountain Phonotephrite Plug

001V92 –110.448 45.993 X Ibex Mountain Phonotephrite Plug

003V92 –110.461 45.987 X Ibex Mountain Phonotephrite Plug

0014V92 –110.530 46.171 X Shields River Road Phonotephrite Plug high LOI

001W92 –110.691 45.871 X X Gobblers Knob Trachyandesite Sill

002W92 –110.449 45.994 X Ibex Mountain Plug

003W92 –110.153 45.973 X Raspberry Butte Sill

005W92 –110.147 45.968 X X Raspberry Butte Trachydacite Sill

006W92 –110.147 45.968 X X Raspberry Butte Basaltic trachyandesite Sill

007W92 –110.147 45.968 X X Raspberry Butte Trachydacite Sill

008W92 –110.147 45.968 X X Raspberry Butte Trachyandesite Sill

009W92 –110.148 45.965 X X Raspberry Butte Trachyandesite Sill

010W92 –110.137 45.981 X X Raspberry Butte Trachyandesite Sill high LOI, low K2

O

011W92 –110.366 46.176 X X Loco Mountain Basaltic trachyandesite Dike

012W92 –110.366 46.176 X X Loco Mountain Trachyandesite Dike high LOI

014W92 –110.362 46.175 X X Loco Mountain Trachyandesite Dike

015W92 –110.362 46.175 X X Loco Mountain Basalt Dike high LOI

016W92 –110.140 46.110 X X Amelong Creek Basaltic andesite Sill high LOI

017W92 –110.145 46.111 X X Amelong Creek Trachyandesite Sill high LOI

018W92 –110.159 46.110 X X Amelong Creek Trachyandesite Sill high LOI

019W92 –110.039 45.974 X X Grosfield Ranch Basalt Sill high LOI

020W92 –110.040 45.978 X X Grosfield Ranch Basaltic andesite Sill high LOI

021W92 –110.043 45.975 X X Grosfield Ranch Basaltic trachyandesite Sill high LOI

022W92 –110.044 45.975 X X Grosfield Ranch Trachybasalt Sill high LOI

024W92 –110.104 45.958 X X Grosfield Ranch Trachyandesite Sill high LOI

025W92 –110.104 45.958 X X Grosfield Ranch Basaltic andesite Sill high LOI

026W92 –110.104 45.958 X X Grosfield Ranch Trachyandesite Sill high LOI

027W92 –110.104 45.958 X X Grosfield Ranch Basaltic trachyandesite Sill high LOI

Status and treatment of samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, Montana.

(Click here to open in Microsoft Excel.)
Appendixes 15

Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, 

Montana. (Click here to open in Microsoft Excel.)

[See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; 

Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; 

P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral]

Field_ID Locality_name SiO2_pct TiO2_pct Al2O3_pct FeO*_pct MnO_pct MgO_pct CaO_pct Na2O_pct K2O_pct

202338 Ibex Mountain 49.00 1.06 14.10 8.51 0.16 7.33 9.97 4.93 3.74

001V92 Ibex Mountain 48.38 1.03 14.59 8.10 0.16 6.48 8.81 6.91 4.46

003V92 Ibex Mountain 49.09 1.03 14.15 8.35 0.17 6.69 8.93 6.71 3.77

014V92 Shields River Road 48.02 1.20 14.70 10.25 0.20 5.24 8.92 8.30 2.42

001W92 Gobblers Knob 57.89 1.22 17.81 5.95 0.11 2.89 5.76 4.41 3.41

002W92 Ibex Mountain

003W92 Raspberry Butte

005W92 Raspberry Butte 62.82 0.87 17.29 5.11 0.08 1.67 4.30 4.11 3.34

006W92 Raspberry Butte 55.64 1.50 16.35 7.41 0.14 4.97 7.24 3.78 2.53

007W92 Raspberry Butte 63.06 0.99 16.90 5.15 0.08 2.05 3.30 4.57 3.47

008W92 Raspberry Butte 57.89 1.05 17.96 6.05 0.11 2.82 6.02 4.45 3.20

009W92 Raspberry Butte 60.25 1.22 17.25 5.98 0.09 2.33 4.77 4.14 3.49

010W92 Raspberry Butte 61.58 1.06 15.24 6.23 0.10 3.72 4.77 6.62 0.29

011W92 Loco Mountain 52.79 1.43 12.89 11.41 0.18 8.64 9.79 1.66 0.84

012W92 Loco Mountain 57.36 1.16 17.54 7.16 0.13 2.67 3.98 4.52 5.02

014W92 Loco Mountain 56.31 1.52 17.39 7.92 0.13 2.82 3.73 6.21 3.41

015W92 Loco Mountain 51.31 1.37 15.23 9.65 0.16 8.00 10.52 2.60 0.74

016W92 Amelong Creek 56.06 1.46 14.88 8.49 0.13 5.77 7.17 3.18 2.36

017W92 Amelong Creek 58.52 1.11 16.99 6.91 0.12 4.45 4.41 4.37 2.65

018W92 Amelong Creek 58.70 1.21 18.47 6.21 0.10 2.31 4.11 5.23 3.12

019W92 Grosfield Ranch 46.32 1.38 14.14 9.97 0.19 6.93 16.03 2.58 2.05

020W92 Grosfield Ranch 54.62 1.15 14.96 9.08 0.11 7.88 6.62 2.96 2.20

021W92 Grosfield Ranch 54.12 1.18 15.84 8.12 0.11 6.58 6.90 4.24 2.49

022W92 Grosfield Ranch 51.12 1.13 14.98 8.20 0.17 4.95 13.49 3.41 2.13

024W92 Grosfield Ranch 57.91 1.26 17.05 8.14 0.11 4.07 3.16 4.69 3.16

025W92 Grosfield Ranch 52.41 1.37 11.32 10.84 0.21 8.95 10.79 2.63 1.15

026W92 Grosfield Ranch 56.59 1.28 17.28 8.16 0.14 3.68 3.77 6.16 2.50

027W92 Grosfield Ranch 54.52 1.28 17.04 7.76 0.11 4.06 6.99 5.18 2.60

Field_ID Locality_name P2O5_pct LOI_pct Total_I_pct Ba_ppm Rb_ppm Sr_ppm Y_ppm Zr_ppm Nb_ppm

202338 Ibex Mountain 1.19 2.76 97.94 3,000 61 3,152 38 190 40

001V92 Ibex Mountain 1.09 1.91 98.04 3,164 89 2,289 34 241 47

003V92 Ibex Mountain 1.11 2.51 98.10 3,215 76 2,415 38 276 56

014V92 Shields River Road 0.74 3.29 98.29 3,371 59 2,916 37 177 47

001W92 Gobblers Knob 0.54 1.39 98.60 2,225 78 1,272 24 229 34

002W92 Ibex Mountain

003W92 Raspberry Butte

005W92 Raspberry Butte 0.41 0.34 98.64 2,014 87 878 19 215 22

006W92 Raspberry Butte 0.45 0.61 98.67 1,634 55 827 23 162 23

007W92 Raspberry Butte 0.42 1.26 98.25 2,405 71 937 19 249 30

008W92 Raspberry Butte 0.43 0.32 98.40 2,287 64 1,204 24 215 23

009W92 Raspberry Butte 0.46 1.59 99.01 1,701 71 936 14 238 24

010W92 Raspberry Butte 0.38 5.29 99.14 627 8 1,170 9 171 23

011W92 Loco Mountain 0.37 2.64 99.32 1,001 24 600 15 144 10

012W92 Loco Mountain 0.45 3.39 98.81 2,272 124 1,019 20 184 18

014W92 Loco Mountain 0.56 1.32 98.80 2,969 56 1,592 26 211 31

015W92 Loco Mountain 0.41 5.75 99.33 799 17 739 22 129 13

016W92 Amelong Creek 0.50 4.81 99.17 1,254 37 740 25 168 19

017W92 Amelong Creek 0.45 5.22 98.90 2,011 46 568 16 191 15

018W92 Amelong Creek 0.52 3.71 98.56 2,734 61 1,033 19 226 20

019W92 Grosfield Ranch 0.41 12.70 98.49 1,929 31 720 18 142 13

020W92 Grosfield Ranch 0.42 8.34 98.77 2,662 35 431 18 140 11

021W92 Grosfield Ranch 0.42 8.13 98.55 4,254 40 716 21 136 18

022W92 Grosfield Ranch 0.41 10.40 98.63 3,176 34 907 19 135 15

024W92 Grosfield Ranch 0.44 4.30 98.41 4,331 54 703 17 179 16

025W92 Grosfield Ranch 0.31 3.11 99.67 1,574 25 480 27 128 12

026W92 Grosfield Ranch 0.44 3.50 98.72 2,446 36 1,382 26 172 16

027W92 Grosfield Ranch 0.45 6.27 98.57 2,921 49 807 18 171 19
16 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana

Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, 

Montana.—Continued

[See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; 

Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; 

P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral]

Field_ID Locality_name Th_ppm Ga_ppm La_ppm Ce_ppm Nd_ppm Cu_ppm Pb_ppm Zn_ppm Sn_ppm

202338 Ibex Mountain 12 164 236 137 122 74 5

001V92 Ibex Mountain 25 161 251 112 184 25 67

003V92 Ibex Mountain 24 152 273 118 23 56 137 5

014V92 Shields River Road 17 151 283 170 197 22 156 2

001W92 Gobblers Knob 32 88 155 42 11 18 88 2

002W92 Ibex Mountain

003W92 Raspberry Butte

005W92 Raspberry Butte 14 57 108 54 71 7

006W92 Raspberry Butte 41 93 36 22 78

007W92 Raspberry Butte 21 76 111 65 28 10 42

008W92 Raspberry Butte 14 62 113 33 47 77 5

009W92 Raspberry Butte 20 48 94 49 9 24 69 7

010W92 Raspberry Butte 19 57 96 40 29 28 74

011W92 Loco Mountain 9 20 60 40 28 32 99

012W92 Loco Mountain 30 47 70 27 24 10 54

014W92 Loco Mountain 26 43 83 62 2 104 2

015W92 Loco Mountain 26 32 67 26 55 30 79

016W92 Amelong Creek 38 76 33 25 14 47 5

017W92 Amelong Creek 9 51 76 46 25 35 62 2

018W92 Amelong Creek 9 73 69 44 65 10 60

019W92 Grosfield Ranch 25 48 78 51 49 29 93

020W92 Grosfield Ranch 16 48 79 49 6 4 58

021W92 Grosfield Ranch 19 36 74 29 22 42 60

022W92 Grosfield Ranch 19 63 72 50 16 17 78

024W92 Grosfield Ranch 10 44 50 22 8 26 80

025W92 Grosfield Ranch 15 18 25 50 52 27 33 111

026W92 Grosfield Ranch 33 80 36 63 20 46

027W92 Grosfield Ranch 17 13 46 68 54 11 25 85

Field_ID Locality_name W_ppm As_ppm Sb_ppm Abd_Pl_phenos Abd_GrnAmph_phenos Abd_BrnAmph_phenos

202338 Ibex Mountain 16 0.4

001V92 Ibex Mountain 6 0.3

003V92 Ibex Mountain 0.2

014V92 Shields River Road 14 0.4

001W92 Gobblers Knob 8 0.2 20 5

002W92 Ibex Mountain

003W92 Raspberry Butte 15 10

005W92 Raspberry Butte 12 0.9 25 1

006W92 Raspberry Butte 2 0.4 3 8 5

007W92 Raspberry Butte 0.7 15 4 5

008W92 Raspberry Butte 0.4 15 3 2

009W92 Raspberry Butte 2 0.6 15 2 5

010W92 Raspberry Butte 28 0.6 4

011W92 Loco Mountain 0.4

012W92 Loco Mountain 0.4 7 2

014W92 Loco Mountain 16 1.2 40 25

015W92 Loco Mountain 2 0 0.6 7

016W92 Amelong Creek 13 0.3 10

017W92 Amelong Creek 0.3 25

018W92 Amelong Creek 0.4 25

019W92 Grosfield Ranch 20 1

020W92 Grosfield Ranch 27

021W92 Grosfield Ranch 4 10 0.3

022W92 Grosfield Ranch 8 0.1

024W92 Grosfield Ranch 15 0.3 3

025W92 Grosfield Ranch 3 0.6 1

026W92 Grosfield Ranch 1 0.2 5

027W92 Grosfield Ranch 19 0.5 1
Appendixes 17

Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, 

Montana.—Continued

Field_ID Locality_name Abd_Bt_phenos Abd_Cpx_phenos Abd_Ol_phenos Abd_Opq TotXtls ClrIndx AgsPl AgsGrnAmph

202338 Ibex Mountain 10 40 TR 3 53 53

001V92 Ibex Mountain

003V92 Ibex Mountain

014V92 Shields River Road

001W92 Gobblers Knob 5 15 3 48 28 3.0 0.3

002W92 Ibex Mountain 3 30 2 44 35

003W92 Raspberry Butte TR 2 27 12 0.8

005W92 Raspberry Butte 1 27 2 0.7

006W92 Raspberry Butte 2 18 15 0.8 0.8

007W92 Raspberry Butte 3 27 12 1.2 0.5

008W92 Raspberry Butte 1 21 6 1.5 0.7

009W92 Raspberry Butte TR 3 25 10 1.5 0.4

010W92 Raspberry Butte 6 1 11 7 1.5

011W92 Loco Mountain 20 4 24 24

012W92 Loco Mountain 3 12 5 1.0

014W92 Loco Mountain 5 70 30 0.5

015W92 Loco Mountain 5 3 15 8 1.0

016W92 Amelong Creek 5 3 18 8 1.0

017W92 Amelong Creek 2 27 2 1.0

018W92 Amelong Creek 2 27 2 2.0

019W92 Grosfield Ranch 15 16 15 0.8

020W92 Grosfield Ranch 15 3 18 18

021W92 Grosfield Ranch 2 2 2

022W92 Grosfield Ranch 15 2 17 17

024W92 Grosfield Ranch 3 6 3 1.5

025W92 Grosfield Ranch 30 4 35 34 1.0

026W92 Grosfield Ranch 1 4 10 5 2.0

027W92 Grosfield Ranch 1 2 1 1.5

Field_ID Locality_name AgsBrnAmph AgsBt AgsCpx AgsOl AgsOpq MgsPl MgsGrnAmph MgsBrnAmph MgsBt MgsCpx

202338 Ibex Mountain 0.9 2.0 1.0 0.20 2.0 3.5

001V92 Ibex Mountain

003V92 Ibex Mountain

014V92 Shields River Road

001W92 Gobblers Knob 0.5 0.7 0.20 4.0 1.2 1.5 3.5

002W92 Ibex Mountain 0.5 2.0 0.10 1.0 3.5

003W92 Raspberry Butte 0.8 0.4 0.05 2.5 2.0 0.4

005W92 Raspberry Butte 1.0 0.10 2.5 2.0

006W92 Raspberry Butte 0.6 0.04 2.0 2.0 1.5

007W92 Raspberry Butte 0.5 0.10 2.5 1.5 2.0

008W92 Raspberry Butte 0.5 0.10 3.5 2.8 1.5

009W92 Raspberry Butte 0.5 0.4 0.10 2.8 0.5 2.0 0.4

010W92 Raspberry Butte 0.5 0.03 1.5 1.0

011W92 Loco Mountain 2.0 0.05 4.5

012W92 Loco Mountain 0.4 0.10 4.0 0.5

014W92 Loco Mountain 0.3 0.05 1.5 1.5

015W92 Loco Mountain 1.0 0.02 2.8 3.5

016W92 Amelong Creek 1.5 0.03 4.5 2.5

017W92 Amelong Creek 0.05 2.0

018W92 Amelong Creek 0.03 4.0

019W92 Grosfield Ranch 2.0 0.01 1.4 4.0

020W92 Grosfield Ranch 0.5 0.02 1.5

021W92 Grosfield Ranch 0.03

022W92 Grosfield Ranch 0.5 0.01 1.0

024W92 Grosfield Ranch 0.04 4.0

025W92 Grosfield Ranch 2.5 0.10 2.0 7.0

026W92 Grosfield Ranch 0.8 0.04 3.2 1.5

027W92 Grosfield Ranch 0.15 2.5
Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, 

Montana.—Continued

[See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; 

Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; 

P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral]

Field_ID Locality_name MgsOl MgsOpq Texture Access_Mnrls XlPl XlGrnAmph XlBrnAmph XlBt XlCpx XlOl XlOpq

202338 Ibex Mountain 1.0 0.40 Hc; E Ap S E S S

001V92 Ibex Mountain

003V92 Ibex Mountain

014V92 Shields River Road

001W92 Gobblers Knob 0.40 Hc; P Ap; Ttn S A A A; S S

002W92 Ibex Mountain 0.50 Hy; P Ap E E; S A

003W92 Raspberry Butte 0.40 Hy; P; T E E S S

005W92 Raspberry Butte 0.40 Hy; P Ap S S A

006W92 Raspberry Butte 0.20 P E S E A

007W92 Raspberry Butte 0.40 P Ap, Ttn E S S S

008W92 Raspberry Butte 0.20 P; I Ap; Ttn E S E A

009W92 Raspberry Butte 0.20 P; Hy Ap E S S S S

010W92 Raspberry Butte 0.10 P; I Ttn (secondary) S S A

011W92 Loco Mountain 0.30 P; Hy Ap S A

012W92 Loco Mountain 0.30 P; I Ap E S A

014W92 Loco Mountain 0.40 Aph; S S S S

015W92 Loco Mountain 0.10 P; I E E S

016W92 Amelong Creek 0.50 P; I E E E

017W92 Amelong Creek 0.40 P; Hc Ap S S

018W92 Amelong Creek 0.60 P; Hc Ap E S

019W92 Grosfield Ranch 0.60 P; I S S E

020W92 Grosfield Ranch 0.05 P; I S S

021W92 Grosfield Ranch 0.20 Aph; I S

022W92 Grosfield Ranch 0.10 Aph; P E A

024W92 Grosfield Ranch 0.60 P; I A S

025W92 Grosfield Ranch 0.60 P; I S E A

026W92 Grosfield Ranch 0.40 P; I S S S

027W92 Grosfield Ranch 0.50 P; I Ap? S A

Field_ID Locality_name Petrog_Com HblClr AltExtnt

202338 Ibex Mountain Includes about 47% anhedral, interstitial nepheline (0.5–4.5 mm, average 

1.5 mm); variably altered to zeolite minerals. Cpx includes distinctive 

emerald-green aegirine overgrowths. 

1+

001V92 Ibex Mountain

003V92 Ibex Mountain

014V92 Shields River Road

001W92 Gobblers Knob Groundmass: clay-altered Pl (0.5 mm), Bt (0.4 mm), Qtz (0.3 mm), 

Hbl (0.3 mm), Cpx (0.3 mm), Opq (0.2 mm). Fine to medium grained. 

Hbl completely altered to Chl.

2+

002W92 Ibex Mountain Groundmass: turbid, moderately devitrified glass. Rock also contains 

7% 0.2–1.2 (0.5) mm nepheline and 2% 0.5–1.2 (0.9) mm cancrinite 

(0.5–1.2 mm, average 0.9 mm). Cpx strongly seive textured.

2

003W92 Raspberry Butte Groundmass: moderately devitrified glass with Pl (0.1 mm), Hbl (0.05 mm), 

Opq (0.02 mm).

Pale tan to tan 1

005W92 Raspberry Butte Groundmass: devitrified intergrowth of Pl (0.1 mm), Hbl (0.01 mm). Some 

secondary Chl.

Pale tan to tan 2

006W92 Raspberry Butte Groundmass: intergranular intergrowth of Pl (0.1 mm), Cpx (0.01 mm), 

Opq (0.02 mm). Green amphibole completely altered to Chl.

Pale tan to tan 3

007W92 Raspberry Butte Groundmass: intergranular intergrowth of Pl (0.2 mm), Opq (0.1 mm). Brown 

amphibole considerably altered to Chl. Green amphibole altered to Chl+Ep.

Pale tan to tan 3

008W92 Raspberry Butte Groundmass: intersertal intergrowth of Pl (0.1 mm), Hbl (0.1 mm). 

Brown amphibole considerably altered to Chl. Green amphibole 

altered to Chl+Ep.

Brown amphibole: Pale tan to tan 

Green amphibole: Yellow green 

to green

2

009W92 Raspberry Butte Groundmass: moderately devitrified glass with Pl (0.1 mm), Hbl (0.05 mm), 

Opq (0.02 mm). Green amphibole altered to Chl.

Brown amphibole: Pale tan to tan 

Green amphibole: Pale green

2-

010W92 Raspberry Butte Groundmass: intersertal intergrowth of Pl (0.1 mm), altered/devitrified glass, 

Opq (0.03 mm). Pl almost completely replaced by Cc. Cpx, completely 

replaced by Cc + clay.

3+

011W92 Loco Mountain Groundmass: intersertal intergrowth of altered (clay+Cc) Pl (0.2 mm), brown 

amphibole (0.05 mm), Chl, moderately devitrifed glass, Opq (0.05 mm). 

Cpx, moderately seived.

3

18 Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana
Appendix 2. Geochemical and petrographic data for samples of sills and plugs peripheral to the Big Timber stock, Crazy Mountains, 

Montana.—Continued

[See table 2 for an explanation of data fields. mm, millimeter; pct, percent; ppm, parts per million; LOI, loss on ignition; Abd, abundance; phenos, phenocrysts; 

Ags, average grain size; Mgs, maximum grain size; Xl, crystallinity. Texture: Aph, aphanitic; E, equigranular; Hc, holocrystalline; Hy, hyalophitic; I, intersertal; 

P, porphyritic; S, seriate; T, trachytic. Accessory minerals: Ap, apatite, Ttn, titanite. Crystallinity: A, anhedral; E, euhedral; S, subhedral]

Field_ID Locality_name Petrog_Com HblClr AltExtnt

012W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole 

(0.1 mm), Opq (0.04 mm). Abundant secondary Cc.

Pale tan to tan 3+

014W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole 

(0.1 mm), moderately devitrified glass, Opq (0.03 mm).

Pale tan to tan 2

015W92 Loco Mountain Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole 

(0.05 mm), Opq (0.03 mm). Abundant secondary Cc; some secondary Chl.

3

016W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole 

(0.1 mm), Opq (0.03 mm). Abundant secondary Cc; some secondary Chl.

2+

017W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.2 mm), brown amphibole 

(0.1 mm), Opq (0.05 mm). Abundant secondary Cc and Chl.

3

018W92 Amelong Creek Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Qtz 

(0.05 mm), Opq (0.02 mm). Abundant secondary Chl.

3

019W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.1 mm), brown amphibole 

(0.05 mm), Opq (0.01 mm). Abundant secondary Chl and Qtz.

3

020W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.1 mm), Hbl (0.05 mm), turbid 

glass, Opq (0.02 mm). Cpx completely altered to Chl+clay. Abundant 

secondary Chl and Qtz.

3+

021W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Opq 

(0.03 mm). Rock may have contained phenocrysts; if so, phenocrysts 

obliterated by alteration.

3+

022W92 Grosfield Ranch Groundmass: aphanitic intergrowth of Pl (0.05 mm), Opq (0.01 mm), 

altered/devitrified glass.

4

024W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.4 mm), Hbl (0.1 mm), 

Opq (0.05 mm). Abundant secondary Cc.

2+

025W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.2 mm), 

Opq (0.05 mm), Cpx (0.1 mm).

2-

026W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.3 mm), Hbl (0.1 mm), Cpx 

(0.1 mm), Opq (0.04 mm).

3

027W92 Grosfield Ranch Groundmass: intersertal intergrowth of Pl (0.2 mm), Hbl (0.1 mm), Opq 

(0.04 mm), devitrified glass (?). Abundant secondary Cc. Hbl in groundmass altered to Chl.

3

Appendixes 19
du Bray and others—Geochemical and Petrographic Data for Intrusions Peripheral to the Big Timber Stock, Crazy Mountains, Montana—Data Series 895

ISSN 2327-638X (online)

http://dx.doi.org/10.3133/ds895